In the production of a granular anionic surfactant, conventionally, a liquid or paste anionic surfactant is used as the starting material and water is removed to carry out drying and granulation, thereby producing the surfactant. Examples of a conventional process for producing a granular anionic surfactant include methods using spray drying such as a method in which a low-concentration slurry having a water content of 60 to 70% by weight is spray-dried taking its viscosity into account (JP-A 55-69698, JP-A 53-39037) and a method in which a high-concentration slurry having a solid concentration of 60 to 80% by weight is spray-dried utilizing the minimum value of the viscosity of a slurry of an alkyl sulfate (JP-A 54-106428).
JP-A 2-222498 discloses a method in which a high-concentration detergent paste raw material having a water content of 20 to 35% by weight is dried using a vacuum thin film drier.
JP-A 5-331496 discloses a method in which a high-density detergent paste raw material is supplied to the inside of a cylindrical casing under vacuum, and a thin film of the high-density detergent paste raw material is formed on the inside wall surface by the tip of a rotatable plate blade and dried rapidly, followed by scraping the dried powder by the plate blade to obtain a flake-like dried product.
In U.S. Pat. No. 5,646,107 and JP-A 2002-508783, a method is exemplified in which a paste is dried and granulated simultaneously by using a flash method or a flash drier.
Also, WO-A 95/01959 discloses a method of producing granules of an alkylsulfuric acid.